Shulou(Shulou.com)06/03 Report--

This article mainly introduces "the importance of Java generics". In daily operation, I believe many people have doubts about the importance of Java generics. The editor consulted all kinds of materials and sorted out simple and easy-to-use operation methods. I hope it will be helpful to answer the doubts about "the importance of Java generics"! Next, please follow the editor to study!

1. Definition of generics

1.1 what is generics?

Generics are about making a class / method capable of supporting many different data types.

1.2 Why use generics?

If there is no generics mechanism, the current encapsulated array classes can only be for int. If you need to target String, you need to create a class and change the type of int in it to String. Later, if you need to add other types, you have to do the same.

With generics, you can have a set of code that can serve multiple types

In the above way, all the types that store data are changed from int type to Object, because Object in Java is the base class for all classes.

If the array held in the class is written as Object [], it can be used to store many different types of data at this time. But to write this way, you have to write a lot of code related to "type conversion".

Public class MyArray {

Public Object [] data = null

Private int size = 0

Private int capacity = 10

Public MyArray () {

Data=new Object [capacity]

}

Public void add (Object data) {

If (size > = capacity) {

Return

}

This.data [size++] = data

}

Public Object get (int index) {

Return data [index]

}

Public static void main (String [] args) {

MyArray myArray=new MyArray ()

MyArray.add (new Person (Zhang San, 18))

MyArray.add (new Person (Li Si, 19))

MyArray.add (new Person (Wang Wu, 20))

Person person= (Person) myArray.get (0)

MyArray myArray2=new MyArray ()

MyArray2.add (new String ("aaa"))

MyArray2.add (new String ("bbb"))

MyArray2.add (new String ("ccc"))

String str= (String) myArray2.get (0)

MyArray myArray3=new MyArray ()

MyArray3.add (new Integer (1))

Integer integer= (Integer) myArray3.get (0)

}

}

It is troublesome to use Object to make do with the "generics" effect.

1. Need to write some code for type conversion

two。 The code for type conversion is error-prone and lacks some necessary type checking.

two。 Generic class

2.1 definition of generic classes

Type parameters are generally represented by an uppercase letter, and the common names are:

E stands for Element

K stands for Key

V stands for Value

N stands for Number

T stands for Type

Smage Upeng V and so on.

2.2 examples of generic classes

When using generics, you need to fill in the actual types of generic parameters when trying to instantiate objects.

After filling in this actual parameter, the actual generic parameter is automatically replaced with String

Java generics can only be reference types. If it is a built-in type, you need a corresponding wrapper class.

Java generics, which are implemented based on Object, are essentially Object [], which helps us to complete the type checksum type conversion.

2.3 instantiation of generic classes

2.3.1 instantiation syntax

Generic class variable name: defines a reference to a generic class.

New generic class (constructor argument): instantiates an object of a generic class

2.3.2 examples of instantiation

Note:

1. The type on the right can be omitted

MyArray2 myArray3=new MyArray2 ()

In the case of the new MyArray2 () object, the type is not explicitly specified in, and the compiler deduces it based on the type on the left side of =.

two。 The type on the left cannot be omitted

The compilation failed after MyArray2 myArray3=new MyArray2 (); / / was omitted

When the compiler deduces, it deduces the right according to the type on the left.

3. Although the one on the right can be written without a type, it cannot be omitted

MyArray2 myArray3=new MyArray2 ()

The above code generates a warning

2.4 definition of generic classes-type boundaries

Type boundaries: when defining generic classes, limit the parameter arguments passed in when instantiating in the future.

Generic parameters the types here should not be filled in casually, but should be constrained according to the actual situation.

Note: there is no boundary E of the specified type and can be regarded as E extends Object

At this point, the study on the importance of Java generics is over. I hope to be able to solve your doubts. The collocation of theory and practice can better help you learn, go and try it! If you want to continue to learn more related knowledge, please continue to follow the website, the editor will continue to work hard to bring you more practical articles!

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